The Role of Enhanced Research Oriented Highway and Foundation Design for Sustainable Development

The Role of Enhanced Research Oriented Highway and Foundation Design & Cons. for Sustainable Development KEYNOTE LECTURE

The Role of Enhanced Research Oriented Highway and Foundation Design for Sustainable Development

J. N. Mukabi1, F. Tatsuoka2, K.Gono3, N. Shimizu4, G. Feleke5, R. Hatakeyama6, W. Demoze7, B. N. Njoroge8, A. Tesfaye9. M. Tadele 10. 1. Construction Project Consultants, Addis Ababa, Ethiopia, 2. University of Tokyo, Japan, 3. Kajima Corporation, Tokyo, Japan, 4. Construction Project Consultants, Tokyo, Japan, 5., 6. & 7. Kajima Corporation Addis Ababa, Ethiopia, 8. University of Nairobi, Nairobi, Kenya, 9. Ethiopia Road Authority, Addis Ababa, Ethiopia, 10. Kajima Corporation, Addis Ababa, Ethiopia ABSTRACT: This paper describes some of the recent advances made in developing research oriented design and construction techniques particularly tailored for developing countries due to the lack of adequate finance and/or limited technological capacity. The paper proposes various design and QC methods considered suitable in ensuring the achievement of a cost-effective and sound structure based on the VE concept. The purposes of innovating such techniques and the results emanating from their practical application are also discussed in terms of cost-effectiveness, performance and environmental considerations.

The study concludes that for purposes of developing appropriate and suitable design approaches and methods of construction, enhanced research is imperative. 1. INTRODUCTION The contemporary Civil Engineer is increasingly faced with major tasks and challenges as a consequence of increased socio-economic activities prompted mainly by population explosion and changing lifestyles. Such tasks and challenges necessitate the construction of larger civil engineering structures such as high rise buildings for increased accommodation, large storage facilities such as liquid tanks as well as highways and bridges that can cater for heavier loads and higher capacity of transportation due to increased traffic. On the other hand, the recently developed environmental policies intended to control degradation can be imposing. Furthermore, the Civil Engineer in developing countries is constrained by lack of sufficient or necessary financial resources and technical capability. Under these circumstances therefore, the Civil Engineer has to constantly develop innovative engineering concepts and methods to face and resolve such challenges in a most appropriate manner. For developing

countries in particular, the methods must take into account factors such as appropriate technology, investment benefit in terms of time, cost reduction of maintenance requirements and most of all, reasonable sustainability. Another perspective is that, geotechnical engineering is founded and enhanced on the basis of three attributes as stipulated by Professor Peck. These attributes include; knowledge of precedents, familiarity with soil mechanics, and a working knowledge of geology. All three can only be developed through enhanced research. The contemporary procedures currently in use in computing design in-put parameters related to bearing capacity, settlement, or factor of safety of slopes for example, are nothing more than the use of the framework of soil mechanics to organize experience derived from case study analysis and experimental work designed to determine appropriate and suitable countermeasures within the available resources.

Pavement design is also quite dynamic in the sense that the concepts are


continually changing as research is enhanced and new data become available. Increased application of theories and principles related to soil mechanics and gotechnical engineering as a basis of research developed for pavement design is to be recognized as a useful approach. 2. MAJOR CONSIDERATIONS

FOR ENHANCED PAVEMENT AND FOUNDATION DESIGN METHODS

2.1 Development of Appropriate

Design Methods Differences exist among Engineers

and the design methods that they adopt for a given set of conditions. This is predominantly due to the fact that most pavement design methods for highways have, as a basis, varying degrees of empiricism that have developed from each individual design agencies correlation to their design method and construction techniques.

This difference can largely be attributed to the lack of a precise and quantitative description as to what constitutes a highway pavement failure or effective performance. The other factors that are contributive include the empirical nature of most designs, differences in testing methods, techniques of evaluation of environmental effects, precision levels of traffic data and history among other factors.

In order to minimize these anomalies and achieve an appropriate design, the aspects depicted in Fig. 2.1 are considered vital.

2.2 Some Examples of Enhanced Research Oriented Bridge Foundation Design In most geotechnical engineering

problems, precise evaluation of deformation properties of soils and rocks is required for the analysis to characterize soil properties relative to prediction of ground deformations and structural displacements under working loads.

On the other hand, recent research on the pre-failure characteristics of geomaterials shows that, in general, geomaterials exhibit non-linear behaviour from a region of very small strains. This implies that linear elastic recoverable behaviour which is of great importance, can only be characterized within a very small range of strains or deformation. Due to the foregoing therefore, many methods to measure the stiffness of soils and rocks are currently in use and/or under development both in the field and in the laboratory. The field tests include several types of seismic surveys, plate loading tests and bore hole e.g. pressure meter tests. The laboratory tests, preferably using high quality least disturbed samples, include dynamic tests e.g. ultrasonic wave tests and resonant-column tests on one hand and static (monotonic or cyclic loading) tests e.g. triaxial compression/extension, plane strain compression, simple shear and torsional shear tests on the other.

However, as can be noted from Figs. 2.3(a) and 2.3(b), considerable differences are often experienced among the stiffness values obtained from these methods. In some cases the difference may be as high as ten-fold or more (Tatsuoka and Shibuya, 1992.

To demonstrate the necessity and importance of employing appropriate methods for measuring as precisely as possible, linear elastic stiffness in the field and laboratory, a number of illustrative examples are introduced.

Fig. 2.2 Shows the general view of the worlds longest and most gigantic Suspension Bridge-the muti-billion dollar

Fig. 2.1 Illustration of Some of the Approaches to provide solutions to Design problems

Advancing Design Method

Improving Material

Characterization Techniques

Enhancing testing and Analytical techniques

Fostering Value Engineering based Cost-Effective

Analysis

Depending on : • Level of Construction Technology • Available Pavement Management


Fig. 2.2 General view of Akashi Strait Suspension Bridge and

Geological Conditions (Tatsuoka et al., 1991)

Akashi Strait (Kaikyo Bridge with a sprawling center span of 1,990m and total length of 3,910m connecting Maiko in Kobe city and Matsuho in Awaji Island. In Fig. 2.3(a, the measured settlement of the bridge’s pier 3P (constructed on sedimentary soft rock of Kobe Formation is compared with that predicted by a linear solution using EPMT (Modulus measured from Pressure Meter Tests value for a depth restricted to the footing diameter. The EPMT values were obtained by linear analysis of pressure-cavity expansion relations from conventional primary loading PMTs. The results show that the EPMT value is very small (‘EPMT 2,890 kgf/cm2 when compared to linear elastic modulus (E10,000 kgf/cm2. EPMT measurements at other structures of the bridge including anchor 1A (constructed on the same soft rock deposit and pier 2P (constructed on a stiff uncemented gravel exhibited virtually similar ratio values by comparison. Based on such experiences, in engineering practice, EPMT values are often converted by using empirical correlations into larger equivalent moduli values comparable to the Plate Loading Test (PLT, EPLT, values obtained from unload/reload cycles (Tatsuoka and Kohata, 1995.

However, there is no sound rationale for this conversion. On the other hand, the

conventional plate loading (PLT) Procedure using a rigid plate often results in an erroneously soft initial relationship between the average plate contact pressure (P)ave and the plate settlement. Such a response could

be due to either the effects of bedding errors at the interface between the plate and the

Fig. 2.3 (a) Relationship between average applied pressure (P)ave and center-line settlement of pier foundation 3P of Akashi Kaikyo Bridge on sedimentary sandstone and its numerical simulation (Siddiquee et al. 1995; Tastuoka et al. 1996b)

Fig. 2.3 (b) PLTs on sedimentary sandstone performed at the bottom level of Anchor foundation IA of Akashi Kaikyo Bridge and their numerical simulations; the unload curve is shown only for PLT S-3 (Siddiquee et al. 1995; Tatsuoka et al. 1996b).

ground surface or the presence of a thin disturbed surface layer produced by excavating and trimming or both. Another reason for the appearance of the S-shaped load-settlement curves is the dependency of drained stiffness on pressure as can be seen from the results of PLTs on a sedimentary soft sandstone plotted in Fig. 2.3(b. The vertical tangent stiffness in the ground largely reflects the tangent Young’s Modulus Etan applied to vertical strain in the ground immediately below the plate. Tatsuoka et al. (1998) observed that if the increase in Etan with σv’ overwhelms the decrease due to strain non-linearity, the tangent ( ) SaveP ∂∂ can increase with (P)ave,


(1) If the instantaneous displacements which occur due to the load from the foundation itself is small enough, the setting of the tower and other super-structures can be adjusted for the foundation displacements during its construction without a serious problem (as it was done). Otherwise, this adjustment become difficult. In case the foundation is displaced largely by the load from the superstructure, the adjustment becomes much more difficult.

(2) If large instantaneous settlements are predicted,

a large creep deformation (and consolidation deformation) of the ground and the associated post-construction continuing displacements of the foundations and the super-structure will also become large. This will have detrimental effects on the superstructure.

(3) If it occurs, a large sag of the girder (the

relative settlement of the girder at the center of the span) will reduce the free height for the sea lane below the bridge associating detrimental effects on the superstructure, and in the extreme case, the deformation of the bridge becomes excessive and visible.

while at larger loads the tangent starts to decrease due to yielding of the ground. Fig. 2.4 shows the general view of the Rainbow Bridge, which is a 798m long suspension bridge with a center span of 570m located adjacent to the Tokyo Port at the seashore of the Tokyo Bay. In designing the bridge, Tomizawa et al. (1987) investigated the inherent anisotropy of a deposit of sedimentary soft mudstone (the Kazusa Group) with a geological age of about 1.5 million years, which now supports the bridge foundations. Fig. 1.5 shows the displacements of Anchor 4A observed upon the construction of the anchorage block (Odagiri et al., 1993). In the design stage, it was considered very important, for the reasons indicated in Table 2A, to estimate precisely the displacement (settlement, lateral displacement and rotation) of the foundations, since this scale of a suspension bridge having foundations placed directly on this type of sedimentary soft mudstone had not been constructed before.

Table 2A

Fig. 1.5 Displacement of Anchor 4A of Rainbow Bridge shown in Fig. 2.43 observed upon the construction of the anchorage block (140,000 ton) and the result of 3D non-linear FEM (dagiri et al., 1993).

Fig. 1.6 Measured distribution of construction of a 140,000 ton anchorage block in sedimentary soft mudstone beneath Anchor 4A of Rainbow Bridge and its FEM simulation (0dagiri et al., 1993; a) settlement and b) lateral displacement inwards to the center of anchor. 3. MATERIAL CHARACTERIZATION

TECHNIQUES

Characterization of pavement materials is predominantly based on standardized procedures. However, in recent years, a great deal of effort has been expended in developing fundamental test methods to characterize the materials response to load and deformation. Although a great deal of these more fundamental tests have not been standardized, engineers are usually encouraged to not only learn from the results obtained from such tests but also to apply the fundamental concepts and principle adopted in innovating them for purposes of enriching their engineering judgment.

Fig. 2.4 General View of Rainbow Bridge (Tomizawa et al., 1987).


3.1 Design of Testing Regimes The necessity to design a

comprehensive testing regime that would characterize the materials to be adopted for subbase construction along Addis Ababa~Goha Tsion Trunk Road Project was prompted by the lack of such materials that would satisfy, within the Projects specifications, the criteria for consistency limits.

In order to minimize the variability factor, pedological series were first determined on the basis of geotechnical soil profile analysis by adopting a matrix sampling method from various pits excavated at different locations of the borrow pits for purposes of characterizing each soil area and profile as precisely as possible.

The relationship between the clay content, clay activity and plasticity characteristics in regard to the geological origin and mineralogy was investigated relative to leaching, laterization and clay mineral coatings of sesquioxides, by varying the degrees of drying of the samples, water temperature applied as well as the chemical characteristics of the water solution through the addition of sodium chloride (NaCl).

The chemical and mineral composition, the size and shape of the soil as well as the orientation of the plates or sheets of the soil mineral were mainly investigated by varying the degrees of saturation and surcharge stress levels through the drying effect as well as variation of surcharge load and varying suction stress levels through the addition of sodium chloride.

On the other hand, the colloidal activity was studied by varying the quality of water in order to investigate the clay activity over a wide range of water content which is the fundamental concept behind colloidal activity. Magnitudes of surface forces which are known to be representative of the colloidal activity as first suggested by Wu (1966) were then analyzed in relation to the slopes of the lines depicted in Fig. 3.1.

Distilled water is usually applied in laboratory testing in order to minimize the

possibility of ion exchange. In this investigation, NaCl was used to study the enhanced ion exchange and analysis was subsequently made on the basis of the comparison between the plasticity characteristics of samples remolded with varying water quality.

3.2 Adaptation of Appropriate

Analytical Techniques Plasticity is a characteristic of all

cohesive soils, and the relationship between the plastic properties of a soil and its constitution and mechanical performance are of considerable importance in soil classification. If a soil is cohesive with an appreciable amount of moisture content, the deformation resistance is reduced and the structure deforms by plastic flow instead of brittle rupture.

In general terms, plasticity index is a function of the amount of clay present in a soil, while the Liquid Limit and Plastic Limits individually are functions of both the amount and type of clay. High plasticity indices are analogous to high water contents whose lubricating effect of the water films between adjacent soil particles tends to reduce the mechanical stability, strength and deformation resistance. This phenomenon is quantitatively illustrated by the following generalized empirical equations proposed by Mukabi et al. (2001c) for tropical soils with a PI < 43.

( )( )( )( )( )( ) EmEm

imc

vmcmc

EUEUimc

vmcmc

qqimcu

vmcumc

PIEE

PIEE

PIqq

βα

βα

βα

+=

∆

+=

∆

+=

∆

max

max

50

50

where, qu=Peak strength determined

from Unconfined Compression (UCS) or Undrained Compression Triaxial (CUTC) Tests, E50=Pre-failure modulus determined from UCS or CUTC tests, Emax=maximum

(3.1)

(3.2)

(3.3)


Youngs Modulus, ∆mc=0.53mcu/mci (Moisture Content Variation Factor), mcu=Ultimate Moisture Content, imc=Initial Moisture Content, PI =Plasticity Index, αq=-0.0123, αEU=-0185, αEm=-0.0362 and

βq=0.535, βEu=0.823, βEm=1.9 are material constants related to strength, pre-failure and Youngs modulus respectively

On the other hand, the empirical relation determined from an extrapolation of the above equations for bearing capacity based on CBR results is defined as : { } 35

564.0027.097.0

≥×−

mCBRPI (3.4)

where CBRm is the measured CBR

value obtained at a density corresponding to 95% MDD in accordance to AASHTO T-180 Method D after 4 days soak.

The above equations were appropriately adopted in the analysis and characterization of the strength and deformation of the soils in question. 3.3 Example of Material Characteri-

zation Approach Analyses of the test results was

undertaken in respect to the intrinsic plasticity characteristics of the geomaterial under the influence of chemical and physical changes on the one hand and the effect of the resulting variation and magnitude of the consistency limit values on the bearing capacity of the geomaterial, on the other. 3.3.1 Influence of clay content - The relationship between clay content and plasticity characteristics is mostly dependent on the geological origin and mineralogy of a geomaterial. Typical results of the soils tested from the Project area exhibit a Liquidity Limit (LL) of approx. 50% (47~52) for various conditions. Plotting the results from the tests conducted in this study in Fig. 3.1 indicates that the level of enhanced clay activity is only 6.4%. This implies that the clay content activity influencing the variation of plasticity

characteristics is very low, hence low potential to swell (ref. to Fig. 3.2). Furthermore, due to the low clay content of the Project material tested, it is considered that the main cause for the initial high plasticity index is the low degree of leaching and laterization due to the low amounts of mineral coatings by sesquioxides which act to suppress the activity of the clay minerals. Since this effect is probably mainly due to the titration of clay particles from the overburden material and weathering, the subsequent geological effect in the post inter-particle state was to increase the sesquioxide influence.

Fig. 3.1 Colloidal Activity Characteristics of Typical Hong Kong Soils (After Lambe, 1962)

Due to the foregoing discussion therefore, it is considered that the clay content and activity effect on the variation of the plasticity characteristics and in particular the plasticity index of the Project geomaterials designated for use as subbase materials is quite negligible.

Fig. 3.2 Swell Potentional of Clay (After Savage et al, 1998)

3.3.2 Influence of nature of soil material -The contribution of the nature of soil materials to the plasticity characteristics highly depends on the shape and structure of

0

10

20

30

40

50

60

70

0 10 20 30 40 50 60 70 80

PLASTICITY INDEX -%

CLAY CONTENT (< 0.002mm) - %

COLLOIDAL ACTIVITY OF SOILS

INACTIVE

NORMALACTIVE

RESIDUAL SOILS

MARINE SILTS

Data from this Study study

SWELL POTENTIAL of CLAY

0

10

20

30

40

50

0 10 20 30 40 50 60 70

Percent , 0.002mm :C

Gro

ss P

last

icity

Inde

x : P

V

HML

L

k = 50

150

200

This Study


the soil minerals in relation to the surface area in contact. In other words, the soil particle orientation and micro-aggregate cluster formation become increasingly important in consideration of the magnitude of consistency limits. The global effect is considered to be related to the ultimate magnitude of the surface of activity of clay minerals in reference to the interaction of sesquioxides. Interpretation of this influence would therefore be related to the genesis, degree of weathering and clay mineralogy. Based on the concepts introduced in section 3.1 the results from the study showed as tabulated in Table B that: Table 3A 3.3.2 Contribution of the chemical composition of the colloid - The practical significance of the liquid and plastic limits lies in their ability to reflect the types and amounts of clay minerals present in the fine fraction (Skempton, 1953). For natural soils, the plasticity index has been found to increase in proportion to the amount of clay sized particles present whereby the relationship is practically linear and passing through the origin as shown in Fig. 3.3. As can be noted from the same figure, very different relationships between the plasticity

index and the percent clay-fraction size are obtained for three clay minerals namely kaolinite, illite and montmorilonite for some temperate-zone clay soils. Wu (1966) suggested that the slope of the lines indicate the relative magnitudes of the surface forces which are representative of the colloidal activity. The active clay characterized by large colloidal activity exhibit plastic properties over a wide water content range. This is generally considered to be the result of the strong interaction between the surface forces and water molecules.

Fig. 3.3 Relation between the Clay Content and the Plasticity Index for some Temperate-Zone Clay Soils and Clay Minerals (After Skempton, 1953)

The results from this investigation showed that although the material exhibited plasticity indices greater than 15, the subsequent variation in plasticity notwithstanding varying conditions was fairly low. This may be attributed to the presence of mica in the silt fraction of the soil as suggested by Ruddock (1967).

This substantiates the fact that the geomaterial along the Project Road is mainly characterized by Kaolinite clay fractions and its plasticity characteristics are hardly influenced by the local history of large seasonal movements.

3.3.4 Influence of exchangeable cations – According to Hough’s (1957) proposal, Sodium (Na), Potassium (K), Calcium (Ca),

PLASTICITY CHARACTERISTICS OF LATERITE SOILS

0

20

40

60

80

100

0 20 40 60 80 100- 2m CLAY CONTENT - %

PLAS

TISI

TY IN

DEX

- %

London ClayllliteHorton ClayKaoliniteCo MontmorilloniteData from this study

Data from

thi t d

i) Virtually negligible variation exists between the plasticity parameters of oven dried and air dried samples. This implies that the clay mineral surfaces do not orient in full contact position as this should have enhanced the degree of saturation and intrinsic localized suction stresses analogous to surcharge stresses whereby the plasticity index of the oven dried sample should have been much lesser than the air dried sample.

ii) The addition of sodium chloride does not seem to

have considerable impact in the plasticity behaviour of the clay minerals tested in this study. This may imply that due to the nature and the structure of the clay minerals, the osmotic suction stress levels are not affected to any such appreciable extent.

iii) Although addition of hydrated lime caused reduction

in the plasticity limits and index to a level below the lower threshold of PI=16, the effect of drying, water temperature in relation to the chemical reaction prompted by addition of CaCo was not apparent. This implied that the nature of the clay minerals was appreciably stable hence drastic variation in the plasticity characteristics was not expected.


Hydrogen (H) and Magnesium (Mg) ions in the montmorillonite mineral exhibit the highest values to Atterberg limits, while those of the Kaolinite mineral exhibit the lowest values. For all minerals, however, the sodium ion is the most exchangeable. In this investigation, sodium chloride (NaCl) and hydrated lime in the form of CaCo3 were adopted as catalistic agents to study to presence and effect of exchangeable ions. The addition of these agents had little influence on the variation of the plasticity values of this material. It was therefore considered that the available exchangeable ions were quite limited and were virtually in an inert state. 3.4 Some Proposed Techniques of Cha-

racterizing Expansive Soils

3.4.1. Methods of testing -(1) In - Situ Methods - A brief description of the testing procedures adopted for characterization of the expansive soils reported in this study is given by Gono et al. (2003a) and in other sections of this paper.

(2) Laboratory Methods - For brief descriptions of both conventional and modified laboratory tests reference may be made to Mukabi et al. (2001c), Gono et al., (2003a) as well as other sections of this paper. 3.4.2 Methods of Analyses- (1) Analyzing Construction History -Computation of total and initial settlement resulting from construction and surcharge of upper layers is considered vital since this influences the characteristics of the roadbed soils and the magnitude of their engineering parameters.

In computing the total settlement, the generalized Eq. (3.5) below was adopted.

∆++

∆= ∑==

ij

KiC

ijo

jii

iC

iijT P

PPe

CHS

01,110log

1 (3.5)

Where, Hi = Thickness of each

layer in cm. The results are presented in Table 3.1

(2) Back Calculation of Induced

Stresses and Strains - Eq. (3.6) was applied for back calculating the initial stress induced by construction machinery and upper layers;

( )[ ]0010 /log PPPe

C ici ∆+

∆= (3.6)

Rewriting Eq. (3.7) we obtain,

)110(0 −∆

= i

kscij P

P σ (3.7)

Where,

ici

iCei ∆∑=

=1σ

It is assumed that the stress is

induced uniformly and that the magnitude of induced stress reduces proportionally with depth. However, the quantitative reduction is average over the depth of each layer as a logarithmic function of the summed reduction in voids ratio (e) and compression Index (CC). The stress induced is computed as a resulting value of the post-construction surcharge.

(3) Application of Deformation Concepts - The deterioration with time of the structural capacity of a pavement has been known to be greatly influenced by the bearing capacity and resistance to deformation of the native subgrade.

Figs. 3.4 ~ 3.6 depict deformation characteristics simulating deterioration in deformation resistance due to the effect of dynamic vibrational loading on a black cotton soil subgrade. These results are part of the JICA Study conducted for the Rural Roads Improvement in Western Kenya. Figs. 3.4 and 3.5 present results after 7 and 14 days cure respectively, while Fig. 3.6 shows the effect of curing period on plastic straining under dynamic loading. Plastic strains were defined as cumulative elasto-plastic axial strains that exist outside the initial yield surface (range of linear elastic and recoverable behaviour). The dynamic


loading was applied directly on the specimens in order to simulate critical conditions whereby the upper pavement structure would have deteriorated drastically leading to a gross loss of its structural capacity. It was derived analytically that the effect of the damaging factor ( )eff

D∆ would reduce proportionally by a factor 7.0

SCφ with the increase in structural capacity of the upper layers. This relation is expressed in e Eq. (3.8).

effDISC

effDR x∆=∆ 7.0φ (3.8)

where, eff

DR∆ =Coefficient of Resulting Damaging Effect, 7.0

SCφ =Structural Capacity Factor, eff

DI∆ =Coefficient of Initial Damaging Effect.

This essentially implies that with a two-fold relative increase in structural capacity of the upper pavement layers, the resilient strain of the untreated soil would be destroyed after about 65,000 cycles of loading and not after about 40,000 vibrations as the results in Fig. 3.15 indicate. It must be noted that the increase is computed in relative ratio terms and not absolute values of the structural capacity. In other words, if the structural capacity was increased from a Resilient Modulus of a

fr

ir MPMtoM 200100 == then 5.0=SCφ and

the resulting multiplier of the positive effect would be the inverse of .SCφ

ea ~ No. of Vibrations Relations

0.0

0.5

1.0

1.5

2.0

2.5

3.0

1,000 10,000 100,000 1,000,000

No. of Vibrations (000's)

ea (%

)

5%lime

Con Aid+ 2% lime

Neat

2%lime

Fig. 3.4 Deformation Characteristics Simulating Deterioration in Deformation Resistance with Dynamic Loading for Black Cotton Soil Subgrade (7 days cure)

ea ~ No. of Vibrations Relations

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

1,000 10,000 100,000 1,000,000 10,000,000No. of Vibrations

ea (%

)

5%lime Con Aid+ 2% lime Neat 2%lime

Fig. 3.5 Deformation Characteristics Simulating Deterioration in Deformation Resistance with Dynamic Loading for Black Cotton Soil Subgrade (14 days cure)

Effect of curing period on Plastic Strain, (ea)p at 84,000 & 336,000 vibrations

0

1

2

3

4

5

6

7

8

0 7 14 21 28Curing Period (days)

Plas

tic S

train

(ea)

p (%

)2%lime:84000 vibes

2%lime:336000 vibes

Con Aid+ 2% lime :84000vibes

Con Aid+ 2% limelime:336000vibes5%lime:84000 vibes

5%li 336000 ib

Fig. 3.6 Deformation Characteristics Simulating in Deterioration in Deformation Resistance with Dynamic Loading for Black Cotton Soil Subgrade in the Post-linear Elastic Zone 3.4.3 Discussion of some test results - (1) Effect of Moisture ~ Suction Variation - Fig. 3.7 depicts the effect of moisture–suction variation on the resilient modulus of black cotton. For purposes of stabilizing the interparticle interaction and achieving reasonable homogeneity, the material was treated with 2% of hydrated lime and thoroughly mixed for a designated period under virtually similar atmospheric conditions for all the tests performed. It can be noted that the relation between the variation in water content and the resilient modulus is practically linear and that a slight change in moisture content greatly affects the magnitude of the resilient modulus.


Mfr = - 497. 04Wc + 5431. 1

0

1000

2000

3000

4000

5000

6000

0 1 2 3 4 5 6 7 8 9 10Wat er Cont ent , Wc ( %)

Resilient Modulus of BCS,M

r(Kg/cm2)

Regr essi onal r el at i on Li near ( Regr essi onal r el at i on)

Moisture-Suction Variation in relation to a Referenc MC of 101% for 2% Line Treated Black Cotton Soil under UC Static Loading Conditions

Fig. 3.7 Effect of Moisture-Suction Variation on Resilient Modulus of 2% Lime Treated BCS

The relation between swelling and bearing capacity of three varying expansive soils is depicted in Fig 3.8. It can be derived that the effect of swell on the bearing capacity depends on the nature of the expansive soil. However, it can be derived that in all cases the tendency of increased swell reduces the bearing capacity and that the critical reduction occurs within the initial range of increased swell.

0

1

2

3

4

5

6

0 5 10 15 20 25Swelling (%)

CB

R (%

)

Frist layer Second Layer Thrid Layer

Fig. 3.8 Effect of Swelling on Bearing Capacity of Expansive Soil

It is well known that one of the

methods of containing swell is to increase the surcharge pressure over a subgrade of an expansive nature. Nevertheless, as can be seen from Fig 3.9 this effect seems to be predominant within the initial phase becoming less apparent with increased surcharge pressure as the swell reaches a residual state. For the material tested in this study, and increase in surcharge pressure of up to about 6 kpa is seen to be effective in containing the swell to a residual state.

0

5

10

15

20

25

0 2 4 6 8

Surcharge (kpa)

Swel

ling

(%)

Frist layer Second Layer Thrid Layer

Fig. 3.9 Effect of Surcharge Pressure on Swell of Expansive

Soil

DS (%) = -4.9815lnλsc + 15.715

05

1015202530

0 5 10 15 20 25Surcharge (kpa)

Swel

ling

(%)

Fig. 3.10 Determining Computational Relation of Swelling Vs. Surcharge Pressure

The results obtained in this study were normalized as presented in Fig. 3.8 in order to arrive at a computational relation expressed in Eq. (3.9) below and proposed in this study. ∆s = As ln λ sc + Bs (3.9)

where ∆s = Swell, As and Bs are material properties related parameters derived as 4.98 and 15.7 respectively for the expansive material adopted in this study. The surcharge pressure is in kpa.

(2) Strength and deformation characteristics of expansive soils under static and dynamic loading conditions. - Subsequent to long–term static loading, the trial sections described by Gono et al. (2003a) were subjected to dynamic loading. As can be noted from Figs. 3.12 to 3.14, the three trial sections were initially subjected to around 61 passes of dynamic loading by use of a loaded dump truck of 1.2 axle


configuration and front and rear axle loads of 4.5 and 9.5 tons respectively. This vehicle was chosen since it represents the most common type of traffic along the project road (Addis Ababa ~ Goha Tsion) as can be seen from Fig. 3.11. Deformation during the static loading stages was measured by use of imbedded pegs (ref. to Gono et al., 2003a), while steel plates were adopted during the dynamic loading stage. In order to analyze the seasonal effects, the sets of both in–situ

0102030405060708090

Bus(1.

2) 1.2 1.22

1.2-22

1.22-2

2

1.22+

2.22

1.2+2

.2

1.22+

2.2

Axle Configuration

Perc

enta

ge

Fig. 3.11 Distribution of Axle Configuration

and laboratory tests were carried out in two stages during the wet and dry seasons under static loading conditions. Dynamic loading was carried out for 20 days subsequent to which the ground response was monitored for 3 days under static loading conditions. Dynamic reloading was then effected for another 4 days after which in–situ measurement of deformation, extrusion of least disturbed samples and material sampling for laboratory testing was undertaken. The deformation characteristics under dynamic loading and static rebound are depicted in Figs. 3.12 to 3.14.

TRIAL1

02468

101214161820

1-1p

1-31

p

8 hr

s-pt

46 h

rs-p

t

9-pt

21-p

t

23-s

24-1

p

24-3

1p

26-p

t

No. of Passes

Def

orm

atio

n (m

m)

T1-1T1-2

Where p pass of truck with known load, Pt public traffic, s static loading

Fig. 3.12 Deformation Characteristics of Expansive Soil under Dynamic Loading (Trial Section 1)

TRIAL 2

0

10

20

30

40

50

60

70

1-1p 1-11p 1-31p 1-61p 8 hrs-pt 22 hrs-pt 46 hrs-pt 96 hrs-pt 9-pt 19-pt 21-pt 22-s 23-s 24-s 24-1p 24-11p 24-31p 24-61p 26-pt

No. of Passes

Def

orm

atio

n (m

m)

T2-1

T2-2

Where p pass of truck with known load, Pt public traffic, s static loading


TRIAL 3

0

5

10

15

20

25

30

1-1p

1-11

p

1-31

p

1-61

p

8 hr

s-pt

22 h

rs-p

t

46 h

rs-p

t

96 h

rs-p

t

9-pt

19-p

t

21-p

t

22-s

23-s

24-s

24-1

p

24-1

1p

24-3

1p

24-6

1p

26-p

t

No. of Passes

Def

orm

atio

n (m

m)

T3-1T3-2

Where p pass of truck with known load , Pt public traffic , s static loading


The facts in Table 3B can be noted from these figures. TABLE 3B

The strength and deformation results

are presented in Table 3.1, while the effect of long-term static and dynamic loading is graphically depicted in Fig. 3.15.

1) The deformation characteristics of the roadbed soils are highly influenced by the nature of the soils and structure of the over laying pavement

2) The rebound behavior is also dependent on the pavement structure

3) In all cases the deformation measured from the outer wheel is larger than that measured from the inner wheel, possibly due to the difference in confining stress as influenced by water infiltration

4) The largest average deformation as recorded for Trial section 2 where ∆d T2= 57mm, while ∆d

T1 = 13.5mm and ∆d T3 =

23mm due to the differences in pavement structure.


It can be noted from there results that long–term consolation and primary dynamic loading tend to enhance the strength and deformation properties of expansive soils. Table 3.1 Strength and Deformation Results under Static and Dynamic Loading of Expansive Soils

Trial

Sec

tion

Post-

Stati

c Loa

ding C

onso

lidati

on

Settle

men

t (cm

)

Aver

age

Peak

De

forma

tion

due

to

Dyna

mic L

oadin

g (cm

)

Aver

age

Peak

reb

ound

Dur

ing P

ost-

dyna

mic

Load

ing u

nder

Stat

ic St

ate

(cm)

Cu Kgf/cm2

E50 Kgf/cm2

Emax

Kgf/cm2 (εa)ELS

(%) x10-4

S1 S2 D S1 S2 D S1 S2 D S1 S2 D

Trial 1 8.35 1.35 0.6 0.44 0.89 1.11 59 76 114 99 128 195 4.8 6.2 9.5

Trial 2 16.72 5.7 0.4 1.33 0.51 0.72 34 49 60 57 81 103 3.1 3.9 5.0

Trial 3 16.8 2.3 0.45 0.28 0.59 0.92 39 83 101 66 105 169 3.9 5.1 8.2

Notes: S1 : Primary Static Loading, S2 : Secondary Static Loading, D : Dynamic Loading

0.20.40.60.8

11.21.41.61.8

0 1 2 3 4Loading Stages

Cu,

(Kgf

/cm

2)

Trial1 Trial2 Trial3 Fig. 3.15 Effect of Long-term Static and Dynamic Loading on Strength Properties of Expansive Soils. Notes: 1 : Primary Static Loading, 2 : Secondary Static Loading, D : Dynamic Loading 3.4.4 Determination of appropriate counter-measures 1) Replacement Method - Tables 3.2 and 3.3 as well as Figs.3.16 and 3.17 show the design and QC criteria developed on the basis of research and adopted for the construction of the Addis Ababa ~ Goha Tsion Trunk Road Project. In determining the necessary thickness tCL to replace the expansive soil, the following equations proposed in this study were adopted.

{ } SPbpPCL xStTt −= (3.10)

The total pavement thickness TP is

expressed as:

vfbPP txRtT += (3.11) And the coefficient of subgrade

structural performance SSP is computed from:

[ ] 5.0//1 edCBRSP eS α= (3.12)

On the other hand, the basic

pavement thickness tPb from Eq. (3.10) is

computed from the following equation.

( ) ( )[ ][ ]P

dPdPPbP

DNCBRCCBRBAt

/logloglog 2+−= (3.13)

Where the roughness factor ( )[ ] 25.02 itif RRRR −= : Ri is the initial roughness

factor and Rt is the terminal roughness factor, tV in Eq. (3.11) is the positive value of the specified tolerence for pavement thickness, AP=219, BP=211, CP=58 and DP=120. The parameter αe in Eq. (3.12) is defined as:

( ) cneeeMLLVCB

ee eA −=α (3.14) where Ae=0.23, Be=0.54, Ce=0.08 are constants and Ve=Annual Average Evapotranspiration in m/year (ref. to Mukabi et al. (2003c), LL=liquid Limit in percentage and Mcn=Natural Moisture Content of the subgrade material expressed in percentage form. All thickness are calculated in mm. Continuous assessment and evaluation of the performance of the sections already constructed by adopting this criteria indicates that the method has so far been quite successful. Table 3.2 Determining Required Capping Layer Thickness (cm) for RE 1 Type Phase II

Codin

g Opti

on Plasticity and Swell

Condition Required Thickness for Different

Subgrade Bearing Capacity

Pl

astic

ity

Index

Swell

(%)

S m

CBR

= 1

CBR

= 2

CBR

= 3

CBR

= 4

4<CB

R<7

A PI>45 Sm>10 140 90 70 60 30

B 35<PI<45 Sm<10 110 75 60 50 20


C PI<35 Sm<5 70 65 55 50 20

Table 3.3 Determining Required Capping Layer Thickness for Varying Quality of Borrow Material

CBR of

Imported material

Required Thickness for Different Native Subgrade Bearing Capacity (cm) for RE 1 Type

Required Thickness for Different Native Subgrade Bearing Capacity

(cm) RE2 Type CBR = 1 CBR = 2 CBR = 3 CBR = 4 CBR = 5 CBR = 6 CBR = 7

15 70 65 55 50 40 30 20 20 60 55 45 40 30 25 20 25 55 45 40 35 25 20 15 30 50 40 35 30 20 15 10 40 45 35 30 25 15 10 10 50 40 30 25 20 10 10 10 60 35 25 20 15 10 10 10

Fig. 3.16 Curves for Determining Required Thickness for Varying Quality of Borrow Material (1<CBRd<4)

Fig. 3.17 Curves for Determining Required Thickness for Varying Quality of Borrow Material (4<CBRd<8)

2) Suction Stress Method - Research for purposes of developing this method is still in the initial stages. The basic idea is to develop a technique of constructing a subsurface drainage layer underlain by a layer compacted to a higher degree in order to induce high but varying suction stresses. The layer is intended to facilitate in directing any excess moisture away from the pavement structure.

3) MC and Swell Control Interface Layer Technique - The on–going research or this subject intends to develop a technique of controlling the moisture content of a subgrade of an expansive nature by systematically and technically imbedding sand columns in predetermined areas or zones. Figs. 3.18 ~ 3.23 present part of the preliminary results that have been obtained in the initial stages of testing. Although definite conclusions can not be derived from these results yet, the trends exhibited from these graphs are distinctly clear. In other words, imbedment of sand interface layers

Fig. 3.18 Effect of Sand Interface Layers Imbedment on Swell under Free Swell Conditions

seams to be effective in reducing swell and increasing the bearing capacity notwithstanding the magnitude of the surcharge pressure.

298 kgf/m2 Surcharge Pressure

0

1

2

3

4

5

6

0 2 4 6 8

Soaking Period,Sp(Hrs)

Swel

l(%)

Plain BCS Single 15mm sand column Double 15mm Sand Column

Notes ♦ Where the results are on the Boundary Limit or

within its vicinity, the Criteria of Clay Activity (Ac) expressed as Ac = 3.6R-2.35 (R=LL/PI) may be adopted or otherwise as directed by the Consultant. For example, should the PI > 45 and Sm < 10, if Ac <1.0 then option B may be adopted instead of Option A or vice versa.

♦ If Ac < 0.75 then maximum swell values of Sm = 15% can be allowed for Options B and C.

♦ Sm represents maximum swell measured uniaxially after 4 days soak and under a standard surcharge pressure of 298kg/m2.

♦ For materials that exhibit excessively high initial rates of swell, the Consultant shall be consulted for further analysis prior to characterizing the expansive subgrade material.

Free Swell

0

2

4

6

8

10

12

14

0 2 4 6 8Soaking Period,Sp(Hrs)

Swel

l(%)



Fig. 3.19 Effect of Sand Interface Layers Imbedment on Swell under 298 kgf/m2 Surcharge Pressure

596kgf/cm2 Surcharge Pressure

0

0.5

1

1.5

2

2.5

3


Sw

ell(%

)


Fig. 3.20 Effect of Sand Interface Layers Imbedment on Swell under 296 kgf/m2 Surcharge Pressure.

Free Swell

0

2

4

6

8

10

12

14

16

18


CB

R (%

)

Plain BCS Single 15mm Sand Column Double 15mm Sand Column

Fig. 3.21 Effect of Sand Interface Layers Imbedment on CBR of Expansive Soils under Free Swell Conditions

0

2

4

6

8

10

12

14

16

18

20

0 2 4 6 8

CBR (%)


298 kgf/m2 Surcharge Pressure

Pla in BC S Single 15mm Sand Column Double 15mm Sand Column

Fig. 3.22 Effect of Sand Interface Layers Imbedment on CBR of Expansive Soils under 298 kgf/m2 Surcharge Pressure.

0

5

10

15

20

25

30

0 2 4 6 8

CBR (%)


596kgf/m2 Surcharge Pressure

Plain BCS Single 15mm Sand Column Double 15mm Sand Column

Fig. 3.23 20 Effect of Sand Interface Layers Imbedment on CBR of Expansive Soils under 296 kgf/m2 Surcharge Pressure.

4.0 RESEARCH BASED METHOD

FOR ENHANCING MECHANICAL STABILITY OF GEOMATERIALS

4.1 Introduction

Most geomaterials in their natural state are usually deficient in one or more of the particle fractions required. Mechanical stabilization of geomaterials is therefore of great importance in achieving a pavement structure which, under loading conditions, is appreciably resistant to lateral displacement. In geotechnical engineering, such composite materials are considered such that the particle size distribution tends towards a correctly proportioned ratio that would yield optimum density and adequate strength to resist stress-induced deformation. Mukabi (2001a) and Mukabi and Shimizu (2001b) further showed that geomaterial which is mechanically stabilized at a ratio tending towards an optimum value may contribute greatly to preventing, to an appreciable extent, the intrusion of subgrade material to upper layers. The detrimental effect of such intrusion on the mechanical stability of pavement layers can be observed from the results presented in section 5.2.2.

Given the importance of determining an optimum batching ratio for mechanical stabilization, Mukabi (2001a) proposed an applicable method. The necessity to develop this method prevailed when, in late 1997 to early 1998, the El-Nino floods caused colossal damage to the tune of approximately US$ 36 million on the pavement structure of the Tana Basin Road Project constructed under the Tana Basin Road Flood Recovery and Rehabilitation Project (ref. to Report to OECF Appraisal Mission for The Additional Loan to Tana Basin Road Project in the Republic of Kenya, March 1999). As a result of this damage, the loan Project funded by OECF (currently known as the Japan Bank of International Cooperation, JBIC) was faced with the problems outlined in Table 4A. As a consequence of the


problems, the Tana Basin Road Project Technical Committee embarked on a research to enhance the mechanical properties of the available geomaterials based on some ideas that had been proposed by Mukabi et al., (1997). The successful implementation of the proposed mechanical stabilization design method would realize cost savings of upto 40% on the hydraulic structures and about 55% on the method of construction [(ref. to (1) Hydrological Review and Analyses for Hydraulic Design of Bridge and Major Culvert Structures and Determination of Areas of Protection Volumes I and II. May, 2000, (2) Engineering Report on the Design and Construction of Reinforced Earth Embankments (The Terre Armee Method) July, 2000; CPC Reports on the Tana Basin Road Project Phase II]. TABLE 4A 4.2 Theoretical Considerations The theoretical point of departure in establishing this method is that soil is regarded as an assembly of particles whose integrated motion can be characterized theoretically by basic concepts and fundamental principles of continuum mechanics and models that consider

probabilistic perspectives of microscopic state and multi-dimensional analysis.

A summary of the theoretical and empirical basis for this method is presented in Figs. 4.1 (a) ~ (h), in the form in which the paper was presented at the World Road Congress (IRF 2001) in Paris.

Fig. 4.1 (a) - (h) Method of Enhancing Mechanical Stabilization of Geomaterials (After Mukabi, 2001a)

4.3 Proposed Method of Determining

Optimum Batching Ratio The mechanical stabilization method,

developed on the foregoing theory, is represented graphically in Figs. 4.2 and 4.3 as well as flow chart 4.1

Important factor Objective of study Develop a method of determining optimum Mixing ratios for geomaterials with different grading characteristics in order to achieve; Enhanced strength (Bearing

Capacity)

Better Compaction characteristics

Greater resistance to wear

Enhanced resilience properties

(c)

(e)

(d)

(f)

(h) (g)

(a) (b)

1. Upon undertaking Case Study Analysis and analytical review of the post-El-Nino hydrological conditions, it was realized that additional hydraulic structures such as bridges and major culverts of appreciable dimensions would be necessary. These structures would necessitate additional funding totaling to almost 30-40% of the total Project cost. However, the economic and financial analysis revealed that investment of such magnitude would not be cost beneficial. Consequently, a cost reduction plan was embarked upon.

2. 3. In order to reduce the costs, a plan to design and

construct bridge approach abutments out of high reinforced soil embankments and reciprocal protection works, was proposed. Nevertheless, due to the non-availability of suitable geomaterials within the project area the preliminary design revealed the cost of reinforcement and protection material would be quite high due to the additional strength and stability required.


Voids ratio e reduces as Batching Ratio Index tends to an Optimum Value. This increase shear Modulus i.e., Gmax =αm(γa).βm.(Am-e)2/(1+e.(σ0)m

Fig. 4.2 Schematic representation of Grading curves generating Graphical Lines Depicted in Fig. 4.3 (After Mukabi, 2001a)

Fig. 4.3 Graphical Representation of New Batching Ratio Method (After Mukabi, 2001a)

Flow Chart 4.1 Proposed Batching Ratio Method (After Mukabi, 2001a)

4.4 Application in Design and Construction

4.4.1 Enhancing engineering properties of reinforced embankments- The concept of reinforcing earth to mainly enhance its strength properties was initially construed in

the early sixties. The oldest amongst the Reinforced Earth methods is the Terre Armee. Its innovation was prompted mainly by the fact that, although earth soil is obviously the cheapest construction material for civil works and is easily available in large quantities, it is usually highly inferior in strength and resistance to deformation. In order to counter this weakness therefore, it was deemed necessary to apply a reinforcing interface that would enhance the ability of the soil-inter particle resistance to deformation and shear stresses. Since the use of the Terre Armee method alone proved to be costly, a design employing a combination of this method and the method of achieving optimum mechanical stabilization proposed by Mukabi (2001a), was devised.

It was noted that, by employing an equal amount of reinforcement strips, the performance of the optimumly mechanically stabilized embankment was superior by about 40% in comparison to the one stabilized using an arbitrarily selected batching ratio. 4.4.2 Highway pavements - As can be derived from Figs. 4.4 to 4.7, the method developed was found to be effective in enhancing the structural capacity of a Fig. 4.4 Batching Ratio Index (BRI) Vs. Voids Ratio (After Mukabi and Shimizu, 2001b) structure in terms of mechanical stabilization, physical strength, deformation resistance and reduction in the rate of subgrade fines intrusion to upper pavement layers.

Determine Grading for both materials

Join the percentages passing for similar sieve sizes for both materials as shown in Fig. 4.3

Plot and join the lower and upper bound of the specification values as shown in Fig. 4.3

Assuming 50:50 Batching Ratio Line as the Phase transformation point, draw Translation Lines from the points where the sieve lines intersect the 50:50 BR Line to the specification Lines (Fig. 4.3)

Mark out the points of intersection at the intersection between the Sieve and Specification Lines (Fig. 4.3)

Calculate individual average Batching Ratios of material from points indicated on Fig. 4.3

Compute overall average Batching Ratio

Calculate the percentages of the respective sieve size for the optimum grading curve

Plot values in grading envelop

Curve of Material A

Curve of Material B

Specification Boundary

Limits

Resultant Curve from Batched


Curve of Material Blended at Optimum Batching Ratio

Optimum Batching Ratio

Stone Ditched Cut outSide drain

Average gradient=4.6%

Rain Water

Propogating Phase

Possiblity of Propagation due to existence of cracks

Area of Intensified Deformation

LongitudinalCracking/Differential Settlement

Horizontal Movementof Poste ObservedPropersity to

Slide alongthis Planedue to Water Lubricationand loss of MechanicalStability of Geomaterialsas a result of waterInfiltration

Fig. 4:11 Cross Sectional Profile of the Slope Failure Area around Sta. 9+275 km

Fig. 4.5 Influence of CBR on Overall Stress-Strain Behaviour (After Mukabi and Shimizu, 2001b)

Fig. 4.6 Influence of BR on UCS (After Mukabi and Shimizu, 2001b)

Fig. 4.7 Influence of BR on E50 (After Mukabi and Shimizu, 2001b) 4.4.3 Stabilization of Slope

Slope failure occurred at a stretch around Sta. 9+310km from the city center of Addis Ababa causing longitudinal cracks within the asphalt concrete and shear failure right through the pavement to the upper part of the subgrade as can be seen from Figs. 4.8 ~ 4.10. The cross-sectional profile of the slope is shown in Fig. 4.11. Due to stringent environmental and cost constraints, a long-

term monitoring, evaluation and assessment method was designed for purposes of determining appropriate countermeasures. The study was undertaken over two seasonal cycles stretching for two years after taking a temporary measure of replacing the pavement material with non-stabilized crusher run material to a depth of approx. 0.5m.

Fig. 4.8 Aerial View of the Longitudinal Crack

Fig. 4.9 Excavated Pit Showing the Line of Longitudinal Crack

Fig. 4.10 Depiction of Line of Shear Failure Plane

Optimum Batching Ratio


1. It was considered that along the main carriageway reduction in bearing capacity and density may have mainly been as a result of decrease in angle of shearing resistance and shearing stress.

2. On the other hand, crack propagation at the joint between the shoulder and edge of asphalt concrete may have been mainly due to reduction in confining stress as a result of increased pore water pressure combined with the effects of dynamic loading due to traffic.

3. The shear failure planes and differential settlement measured and observed indicate that the failure tendency may have been towards a critical state for total sliding as excitement due to dynamic traffic loading increased.

Fig. 4.12 Borelogs Portraying the Pavement Structure and Underlying Soil Profile of the three pits excavated at the Slope Failure Area. The log profiles of the pavement structure and underlying soil profile is depicted in Fig. 4.12, while Fig. 4.13 indicates the points at which comprehensive investigations were carried out. The results in Figs. 4.14(a) and (b) depicting transversal and vertical movement respectively, show that movement in both directions is cumulatively continuous. Fig. 4.13 Plan View of the Designated Field Survey and Monitoring Points

Fig. 4.14 (a) Transversal Movements of the Failed Area (b) Verticle Movements of the Failed Area.

Based on the initial analysis of field

and laboratory data as well as field monitoring, it was considered that the failure may have been prompted initially by increased moisture content due to water infiltration and subsequently by the combined components of dynamic loading and pore pressure increase effects. These effects may have then resulted into the derivations indicated in Table 4B.

TABLE 4B Various countermeasures including

reinforcement of slope embankment, construction of retaining wall, reduction of

The Role of Enhanced Research Oriented Highway and Foundation Design & Cons. for Sustainable Development KEYNOTE LECTURE Page 20

1. Retaining a substantial proportion of their strength even with increased saturation levels.

2. Reducing tremendously the surface deflection of the layers under loading.

3. Increasing resistance to erosion due to the scouring effect of water flow.

4. Increasing resistance to contamination by materials in underlying or supporting layers that are not stabilized.

5. Increasing the effective elastic moduli of the composite pavement structure.

gradient of slope, improvement of subsurface drainage conditions, blanket and loading works etc. were preliminarily considered. However, the most cost-effective option was considered to be cement and/or lime stabilization of the materials in-situ, whereby only material for the new base course layer would be imported. It was considered that this method would be effective in enhancing the intrinsic properties of the road materials and pavement layers as stipulated in Table 4C.

TABLE 4C

The design of the testing regime was

fundamentally meant to simulate normal and critical conditions as are likely to occur in the field and enable the determination and establishment of appropriate and cost-effective measures to the existing problem.

The preliminary results tabulated in Table 4.1 and graphically depicted in Figs. 4.15 to 4.17 show the effectiveness of integrating chemical stabilization with the newly proposed mechanical stabilization

method (Mukabi, 2001a). The testing was conducted after 7 days of curing and the materials were treated with 5% cement. Further comprehensive testing is on-going.

2.00

3.00

4.00

5.00

6.00

7.00

8.00

9.00

0 1 2 3 4Unc

onfin

ed C

ompr

essi

ve S

treng

th,q

u(M

pa)

4:1 , Unsoaked 4:1, Soaked3:2,Unsoaked 3:2,Soaked

1. Plain Aggregate 2. Chemically Stabilized Aggregate3. Chemically & Mechanically Stabilized Aggregate

Fig. 4.15 Effect of Stabilization on UCS

20

25

30

35

40

45

50

0 1 2 3 4

Ang

le o

f Int

erna

l Res

ista

nce

(%)

4:1 , Unsoaked 4:1, Soaked

3:2,Unsoaked 3:2,Soaked

1. Plain Aggregate 2. Chemically Stabilized Aggregate3. Chemically & Mechanically Stabilized Aggregate

Fig. 4.16 Effect of Stabilization on Angle of Internal Resistance

2000

3000

4000

5000

6000

7000

8000

9000

0 1 2 3 4

Res

ilien

t Mod

ulus

,Mr(

Mpa

)

4:1 , Unsoaked 4:1, Soaked3:2,Unsoaked 3:2,Soaked

1. Plain Aggregate2. Chemicaly Stabilized Aggregate3. Chemically & Mechanicaly Stabilized Aggregate

Fig. 4.17 Effect of Stabilization on Resilient Modulus

Table 4.1 Strength and Resilient Modulus Results of Chemically and Mechanically Stabilized Aggregate

Serial No.

Batching Ratio

Soaking Condition

Unconfined Compression Strength (Mpa) Angle of Internal Resistance (0) Resilient Modulus (Mpa)

Plain

Aggregate

Chemically Stabilized Aggregate

Chemically & Mechanically

Stabilized Aggregate

Plain

Aggregate




Plain

Aggregate




1 4:1 Unsoaked 2.69 3.26 4.23 23.2 27.9 36.5 2838 3405 4461

2 4:1 Soaked 2.62 3.21 4.21 23.6 28.1 36.5 2825 3390 4441

3 3:2 Unsoaked 5.07 6.01 8.14 29.4 35.1 44.7 5466 6559 8592

4 3:2 Soaked 3.79 4.16 6.13 25.8 31.2 40.5 4032 4839 6339

The resilient modulus was computed from the empirical relation first proposed by

Mukabi (2000) and modified by Mukabi et al. (2001f) generally expressed as follows.


[ ] S

RFtrquqqur xfxBqAM α+= (4.1)

where qu

rM =Resilient Modulus, Aq =6.33 and Bq = 179 are constants,

)(6.1)( untreatedu

qq treatedutr =α expresses the quantities effect of treatment and

( )optII

cr

sopt

sRF BRBRRff −•−= is a strength and

moduli ratio parameter derived from the influence of mechanical stability.

5. PROPOSED APPROACH OF EVALUATING THE STRUCTURAL CAPACITY OF AN EXISTING PAVEMENT STRUCTURE

For purposes of effectively evaluating as precisely as possible, the structural capacity of the existing pavement structure, it is extremely vital to undertake a comprehensive study based on historical environmental data and case study analysis. A proper evaluation and analysis of the road surface deficiency, effects of seasonal changes, damaging effect associated with traffic loading and the quality of the existing material and pavement layer thickness is almost imperative in any case. 5.1 Evaluation of Road Surface

Distress In order to achieve the objective of

the exercise of evaluating and assessing the extent of structural damage by referring to the surface distress condition, it is necessary to not only undertake a proper assessment but also to modify the existing concepts and perspectives. For example, Mukabi and Shimizu (2000, unpublished), introduced a depth/diameter ratio factor to facilitate the assessment of road surface distress associated pavement structural capacity in relation to the magnitude of chuckholes. Fundamentally therefore, the following criteria for chuckholes was developed. When ch/dch > 0.5, then Pc = 1.2xP, where, ch = diameter of chuckhole, dch =

depth of chuckhole, PC = Corrected Patching Area value in m2 per 1000m2 area and P = Bituminous Patching area expressed as :

( ){ }[ ] CPSIRDSvSdfAP −−−+−=2

01.0238.1)1log9.1 (5.1)

Where, Asdf = Surface distress factor depending on pavement type and structural design, Sv = Slope variance, RD = Rut Depth in cm for both wheel tracks measured with a 3m straightedge, PSI = Present Serviceability Index, C=Lineal measurement of cracking per 1000m2 area. 5.2 Proposed Method of Evaluating

Structural Capacity 5.2.1 Introduction

Fig. 5.1 Intrusion of Subgrade Material into Upper Layers of a Pavement Structure

Intrusion of underlying material into upper layers of a pavement structure tends to occur in most flexible pavements as depicted in Fig. 5.1. In order to evaluate quantitatively, the impact of subgrade fines intrusion, a laboratory testing regime to simulate the field conditions was designed by the Construction Project Consultants (CPC) and Kajima Corporation, Ethiopia Research Group. The samples were extruded from the field as schematically illustrated in Fig. 5.2 and various relevant tests performed accordingly.


Fig. 5.2 Mode of Sampling for Laboratory Sieve Analysis

5.2.2 Impact of expansive subgrade material infiltration - Depending on the nature of the subgrade, topography of environment and seasonal changes, intrusion of native subgrade material into overlying layers of the pavement structure can be rampant. Such intrusion usually results in the ultimate degradation of the structural capacity of the respective layers. This is as a consequence of drastic changes in the material characteristics of the upper layers resulting into the deterioration of the cohesion intercept (c), the angle of internal friction or resistance () as well as mechanical stability, factors which affect the bearing capacity and deformation properties of the pavement structure.

Part of the results demonstrating the seriousity of this effect are presented in Figs. 5.3 ~ 5.5. It can be seen from Fig. 5.3 for example, that intrusion of small quantities of black cotton soil fines into the high quality mechanically stabilized crushed aggregate material cause a virtually linear reduction, in bearing capacity, which is dependent on saturation levels and effective surcharge pressure. However, an intrusion quantity greater than 10% of the black cotton soil results in a drastic drop in bearing capacity notwithstanding the soaking or surcharge pressure conditions except for the unsoaked specimen whose drop in bearing capacity was from CBR=100% to CBR = 45%.

The drastic change in mechanical stability properties of the blended material can be observed from Fig. 5.4, while the intrusion influence on the swelling potential can be seen from Fig. 5.5. It can be derived that, for this crushed aggregate base course material mechanically stabilized at a 4:1

(0~40mm : 0~5mm) ratio, a percentage increment of approximately 10% high plasticity expansive soil intrusion is probably the threshold for bearing capacity, deformation resistance, mechanical stability and swell. Further research on this influence in relation to different quality aggregates as well as varying quality of intrusion material and mechanical stabilization ratios is currently being undertaken by the Research Group.

Fig. 5.3 Influence of Black Cotton Soil Intrusion on the Bearing Strength of Mechanically Stabilized High Quality Base Course Material

7575

6363

37.5

37.5

1919

9.5

9.5

4.75

4.75

2.36

2.36

1.18

1.18

0.42

50.

425

0.25

0.25

0.15

0.15

0.07

50.

075

0

10

20

30

40

50

60

70

80

90

100

0.01 0.1 1 10 100SIEVE SIZE (mm)

TOTA

L P

ER

CE

NT

PA

SS

ING

(%)

Dark Clay Gray Clay

Red Clay Base Course

Blanded ( Base course : black cotton, 9:1 ) Blended (Base course : black cotton , 7:1 )

Blanded ( Base course : Black cotton, 4: 6) Fig. 5.4 Sieve Analysis Depicting Variation in Mechanical Stability due to increase of Black Cotton soil in M.S Base Course Material

Fig. 5.5 The influence of Black Cotton Soil Intrusion on Swelling Potential

0

20

40

60

80

100

120

140

160

0 10 20 30 40 50

Black Cotton soil Content

CB

R(%

)

Unsoaked

Soaked for 96hrs with out surcharge

Soaked for 96hrs with 5kg surcharge




0

2

4

6

8

10

12

14

16

18

0 10 20 30 40 50

Percentage swell

Black Cotton soil Content

UnsoakedSoaked for 96hrs with out surchargeSoaked for 96hrs with 3kpa surchargeSoaked for 96hrs with 6kpa surchargeSoaked for 48hrs with 3kpa surchargeSoaked for 48hrs with 6kpa surcharge


5.2.3 Evaluation of structural deficiency - In evaluating the structural capacity and/or deficiency of an existing pavement structure, it is imperative to analyze both dynamic and static wheel loads.

For purposes of quantitatively evaluating the relative damaging effect emanating from increased axle loading, a growth Intensity factor (IGf) related to cumulative ESAL was introduced by Mukabi (2002a) as expressed in Eq. 5.2.

i

n

iiGf xESALGI ∑

=

=1100

1 (5.2)

where G represents the relative

growth rate and ESAL is the equivalent single axle load. The major structural components that contribute to the pavement structural deficiency include the relative damaging effect as a result of increased ESAL, degradation in the quality of materials prompted by environmental factors and variation in pavement layer thickness through loss of aggregates and deformation. The concept of remaining life is introduced in this section for purposes of evaluating the total structural capacity required to support the overlay traffic over existing subgrade conditions subsequent to determining the effective structural capacity of the existing pavement under similar conditions. The remaining life factor FRL, is determined as a function of the components contributing directly to the structural deficiency of the pavement i.e.,

fsd = FRL = f (RDeff, ∆PSF, ∆RSF, ∆PSI) (5.3)

In consideration of the reciprocal

effects of the parameters in this equation it is considered that the following relation proposed in this paper, be adopted in determining FRL.

fsd = FRL = 1 - [RDeff. x (1-∆rf)] (5.4)

where, ∆rf= Combined contribution of other factors deemed to have had a reciprocal effect on the magnitude of the damaging effect. An example of application of this concept is given by Mukabi et al. (2003c).

The factor fsd is considered to be the structural capacity depreciation factor and is applied as a multiplier to the main structural components i.e. strength and effective layer thickness in determining the ultimate effective structural capacity of the existing pavement in its current status.

5.2.4 Application of the structural thickness concept - Examples of the quantitative changes in pavement structural thickness, which is defined as the effective thickness that acts structurally, and the direct impact on the bearing capacity have been discussed by Mukabi et al. (2003c).

By application of the Intensity Factor If proposed by Mukabi et al. (2002a), evaluation of structural thickness and the corresponding deterioration is considered possible. The formulae are presented below.

( )sdffEP

Df txSxR

TTI ∆∆+= 1 (5.5)

where, TD = Design Thickness, TEP=Measured Thickness of the Existing Pavement, Rf =Roughness Factor expressed

as ( )

25.02

−=

it

iRR

RRf where, Rf = roughness

Factor, Ri=Initial Roughness Value, Rt= Terminal Roughness Value, ∆Sdf=Rate of Surface Distress Depreciation Factor, ∆ts=Time lapsed since the previous study or survey was undertaken.

The resulting thickness ∆TR is then determined from the relation;

∆TR = TMf—T0 (5.6)

where TMf = Modified Thickness.

The following equations are then applied in determining the deviation and variance factors in the form:-


( ) ( )

1

505022

2−

−−−=∑ ∑

n

TxITxIS

DfDf (5.7)

While,

( )AV

RTxSVC

∆=

100.5.02

where, AVRT∆ = Average

Resulting Thickness is computed as,

AVeT∆ =

n

TRi

n

i∑=

∆1 (5.8)

5.2.5 Quantitative analysis of required structural capacity - The significant importance of adequate structural capacity is well defined in the generalized basic design equation which delineates the major components necessary to ensure appreciable performance, serviceability and reliability. The generalized state of this relation is expressed in Eq. (5.1) below. Wt = f (∆PSF, ∆RSF, ∆CRF, ∆PCF )x Rf (5.9)

where, ∆PSF=Pavement structure factors i.e. upper pavement layer thickness, ∆RSF = Roadbed soil factors i.e., soil resilient modulus, ∆CRF =Climate related factors i.e., drainage and evapotranspiration coefficients, ∆PCF = Pavement condition factors i.e., terminal serviceability index, Rf = Reliability factor, where Rf = Wt/WT > 1 and WT is a traffic prediction factor.

As can be noted from this equation, the three components ∆PSF, ∆RSF and ∆PCF are fundamentally related to the structural capacity, whilst ∆CRF and Rf are factors that predict the possible damage or deterioration that can be caused on the pavement structure relative to its structural capacity.

In analyzing the required structural capacity for an overlay design for example, temperature and seasonal effects should be given serious consideration as discussed by Mukabi et al. (2003b, 2003c). In order to simulate both seasonal and temperature effects on the strength and modulus of

deformation of asphalt concrete, the method proposed and described by Mukabi et al. (2003a) was adopted. The results indicate that the temperature of the AC may be lower by about 30% in the wet season when AC temperatures are greater than 45oC,compared to the AC temperature in the dry season. Essentially therefore, a correction factor of 1.3 should be adopted to correct the temperatures in relation to the seasonal effects. For AC temperatures between 35oC and 45oC, a correction factor of 1.15 may be applied, while AC temperatures lower than 35oC may not require correction. It is also recommended that these correction factors be adopted during deflection testing in varying wet or dry seasons, particularly in hot tropical areas. On the other hand, it was determined in this study that on the average, the AC temperature is almost always 1.4 times the ambient temperature, with slight variations between the wet and dry seasons.

6 SOME PROPOSED RESEARCH

BASED PAVEMENT DESIGN PRINCIPLES

6.1 Overlay Design The importance of applying advanced and appropriate analytical techniques can be observed from Figs. 6.1 and 6.2. Based on comprehensive research undertaken under JICA funding, it was found that the equation proposed by the Asphalt Institute tends to over-estimate the structural capacity of the existing pavement structure as a result of the lack of considering various factors related to environmental and structural depreciation with time, as input parameters in the equation. Based on elastic moduli results from advanced testing of various geomaterials and by inputing environmental and structural depreciation components that would characterize the elastic behaviour of the respective geomaterial, Mukabi (2000) proposed the modified equation shown in Figs. 6.1 and 6.2.


(a) OVERLAY DESIGNS

Conventional Approach Recently Developed Approach

( )adCBRr MPCBRM 3.10= ( ) ( )

+

=∆MrCBRCBR

M

CBRrCor

rMrMrA

MM

rlnln

ln5.9738.0

(proposed by the Asphalt Institute) T1

AC AC T2AC

T1BC BC

T2BC

T1SB

SB

T2SB

Note - AC = Asphalt Concrete - T1

AC = T2AC

- BC = Base Course - T1BC < T2

BC

- SB = Subbase - T1SB < T2

SB (b) RECONSTRUCTION DESIGNS Conventional Approach Recently Developed Approach with Example

=2/

0782.0

max 139mckgfu

q

scSCdg

j xexmE α

where

( )5050 / aCE ε= ( ) 78.0189.1 42.0 =−= uscdg qα

Conventional And

( )( ) 21 /3.385.0/ cmkgfqmqqm Rq

RuSCu === −

T3AC Reference strength I denotes trial

section No. and j denotes layer No. Applying Eqn. (2.1)

T4AC

T3BC

T4BC

069.011max 85.078.0139 xexxE =

211

max /99 cmkgfE =

T4SB

T3SB

( ) ( 25011max /

0104.0099.0cmkgfx

qEE n

qu

ψ−

=

Where

806.02 == Ru qqψ

and 699.0

0104.066.20996.0(24811

max xx

E−

=

211max /681 cmkgfE =

- T3

AC = T4AC n=2.24e-0.117q

u=1.663 since 2<qu<15kgf/cm2

- T3BC > T4

BC

- T3SB > T4

SB Fig. 6.2 Comparative Example of Anomalies that Prevail in Overlay and Reconstruction Design as a Result of Adopting the Conventional Approach 6.2 Reconstruction Design

As demonstrated under section 2.2 of this paper and as can be derived from Figs. 6.3(a) ~ (c), the importance of determining Small strain stiffness is demonstrated. TABLE 6A Fig. 6.3 Importance of Considering Small Strain Measurement (After Mukabi and Tatsuoka, 1999c)

Fig. 6.1 Mode of Determining Correction Factor and Equation for Resilent Modulus Determined from CBR Results

Basis for Developing Elastic Limit Strain Theory and Equation As can be derived from the preceding discussions, the determination of the linear elastic range of geomaterials defined as the region of the initial yield surface within which the behaviour of the geomaterial is virtually linear elastic and recoverable, is of paramount importance. Consequently, based on long-term research undertaken since 1991, Mukabi proposed the following equation for estimating the linear elastic range. ♦ ELS Equation

( ) ( )( ){ } (%)

max

50

Aija

ijELS

ijaij

ELSa +=

εφε

ε Where ESLφ is

a function of the level of max)( aε and A is a constant

depending on the physical properties of the geomaterial. For most clays φ is defined as,

462max

50 xEE

ELS

=φ and,

A = 603 where the curve is considered to be positive in all quadrants.

(a(a)

(b)

(c)

0 0.001 0.002 0.003 0 004 0 005


7 APPLICATION OF CONSOLIDA-TION AND SHEAR STRESS RATIO CONCEPTS IN FOUNDATION DESIGN AND CONSTRUCTION

7.1 Introduction Laboratory tests are primarily carried out for purposes of obtaining engineering parameters which can be directly applied to conditions in the field. Such an exercise would not only provide parameters for design and construction quality control but also an insight into the fundamental processes which affect the field behaviour. Laboratory tests can for example, employed to investigate how strength and stiffness develop during large strain consolidation and how this behaviour is dependent on various factors such as loading rate and direction, principal stress rotation in relation to location within the foundation etc. However, the precision of adopting these results involves an analytical approach that would be appropriate in simulating as accurately as possible the actual field conditions.

Furthermore, precise determination of such parameters for natural clays usually requires high quality sampling and testing techniques for a reliable laboratory investigation. This translates to high costs and long time durations for performing the tests.

In developing countries, both affordability and accessibility to high quality equipment are major curtailing factors to realizing this aim. This situation therefore necessitates the development of empirical methods that can aid in providing estimated parameters that are reasonable enough for the design and modelling of foundations bearing civil engineering structures.

7.2 Derivation of Consolidation and

Shear Stress Ratio Concepts The method that is briefly described

in this paper is based on that proposed by Mukabi and Tatsuoka (1999b) which modified some aspects of the Critical State Soil Mechanics (CSSM) theories. This was

prompted by their (Mukabi and Tatsuoka 1992) investigation into the effects of consolidation stress ratio and strain rate on the peak stress ratio of clay which concluded that the shear stress ratio (q/p’)max, increases as the consolidation stress ratio

''acK σσ= decreases.

Their study also confirmed that the shearing stress ratio at failure Kf is a function of the initial consolidation stress ratio and that it decreased proportionally with decreasing Kc parameters. The definition of the stress parameters applied is presented in the p’~ q plane in Fig. 7.1.

Fig. 7.1 Definition of Stress Parameters in p’~q plane (After Mukabi and Tatsuoka, 1999b)

An arbitrary value defined as the q/p’

at the Critical State Line (CSL) is presented as ηmax = (qm/p’m) = (q/p’)max, for a sample sheared at εa = 0.01%/min. When Kc = 1. The stress ratio at the conventional CSL, but not necessarily under ultimate conditions, is defined us μ = (q/p’)cs, while the degree of Overshooting (DOS) factor defined in relation to the CSL is computed as DOS = (ηmax/ μ)- 1.0. The reconsolidation stress ratio is given as ηc = (q/p’) and defines the distance to failure from the current consolidation state of stress as μ – ηc (qmax)A depicts the peak value of shear strength incase the undrained shear stress path had started from point A delineated as the intersection of the undrained shear stress path of Kc (I) specimen and Kc < 1 reconsolidation stress path, instead of starting from point S. Consequently, the Consolidation Stress Ratio (CSR) factor described as the distance to failure from the


current consolidation state of stress, is given by CSR = ηc/ μ.

In modelling the behaviour of clay within a modified framework of CSSM, a method of erasing the effects of loading rate in relation to φ’ as a function of CSR, was established. The relationship of the CSR function denoted as δCSR = [(eCSR)] and ∆φ’; a function of the normalized angle of internal friction (φ’/qmax) in reference to that of Kc = 1 i.e., ∆φ’ = ∆Aφ/∆Iφ (where A stands for Anisotropic and I refers to Isotropic ) is represented by a linear equation derived from Fig. 7.2 in Eq. 7.1 as :

φφδφ BA CSR +=∆ ' (7.1) where, Aφ=0.1628 and Bφ=7.963 are constants.

Fig. 7.2 CSR Function (δCSR) - φ’/qmax Relations The linear relations in Fig. 7.2 are

practically the same hence show no dependency on strain rate (0.002< εa < 0.5%/min) suggesting that the equation uniquely relates CSR and φ’ and can be applied in developing further mathematical relations without particular reference to strain rate effects.

Having characterized the effects of loading rate into a generalized state, a method of unifying the behaviour of anisotropically consolidated clay into a coherent form was considered. A κ-function which relates the φ’ determined from various tests performed by applying different CSRs was defined as κ=φ’/ ηqm. This relationship was found to be virtually constant and related closely to a reference line considered

to be analoguos to a modified CSL (i.e., φ’ constant when Kc = 1). Consequently, this led to the derivation of the various generalized state relations which are discussed in detail by Mukabi and Tatsuoka (1999b) and Mukabi (2001d).

In order to compute φ’, a relation between the invariant stress ratio (∆SR) and the angle of internal friction was derived from linear regressional analysis of experimental data on various clays as shown in Fig. 7.3. This yielded the empirical equation expressed below.

SRSRSR BA +∆='φ (7.2)

where ASR = 23.6, BSR = 1.3 are constants and ∆SR = (q/p’)max is the invariant stress ratio variable.

Fig 7.3 Regressional analysis of φ’ ~ ∆SR Relations 7.3 Application During Multi-stage

Construction of Soft Clay Embankments And Foundations

7.3.1 Functions and parameters related to the concept of loading rate - Due to the importance of incorporating the analysis of the effects of loading on foundations and embankments of clayey geomaterials during modelling and design, Mukabi and Tatsuoka (1999b) developed a relation between the stress ratio at failure ηmax (q/p’m) and axial strain rate (εa) expressed in a generalized state as :

ηη εη BA SRan += 1max (7.3)


where constants AηI and Bη were determined as Aη

A = 0.037, BηA = 0.858 and AI

η=0.043, Bη

I=0.76 (Superscripts denote; A : Anistopic; I: Isotropic; SR: Strain Rate). Based on comprehensive analysis of various clays subjected to different axial strain rates and on Eq. 7.3 the following co-relations were derived:-

ηη εε

ω BA RSRa

ASRa

nSR +

= 1 (7.4)

and

RSRASRmax

1max ηωη •= −

(7.5)

where ωSR is a strain rate function and superscripts ASR and RSR denote “Applied Strain Rate” and “Reference Strain Rate” respectively. 7.3.2 Functions and parameters based on SHANSEP consolidation - As was discussed by Mukabi and Tatsuoka (1999a and 1999b) and Mukabi (2001d), the “intact” specimen exhibits much more superior behaviour in comparison to the specimens reconsolidated applying the SHANSEP method. It was also derived that the higher the stress level of the consolidation stress ratio ηc=(qc/p’c), the more the structure is destroyed through remoulding. This implies that specimens reconsolidated by applying the SHANSEP method can not be representative or correctly simulate the in-situ conditions. Consequently, correction factors have to be applied on the parameters determined adopting such a method. Based on the concepts of consolidation and shear stress ratio functions, the following correlations for qmax, pf” and φf” were derived.

NCS

OCS

NCSNCSc

OCScNCS

NCSOCS

CSRAK

qKqmax

max

0

max0max

' ν

ν

σσ

φηη

•

••

−

•= (7.6)

NCSc

OCSc

NCSNCSc

OCScNCS

NCSf

NCSOCSf P

p

CSRAK

pKp

''

'

''

0

0 •

••

−

•=

φηη

(7.7)

••

−

•=

NCSNCSc

OCScNCS

NCSf

NCSOCSf

CSRAK

K

'0

0 ''

φηη

φφ (7.8)

where, subscript f denotes failure,

superscript OCS and NCS denote Over Consolidated and Normally Consolidated under the SHANSEP method. 7.3.3 Functions and parameters related to the concept of aging - Aging is considered to constitute mainly of two components; namely secondary consolidation associated with creep ( )0' =∂∂ taε and thixotropy defined as a gain in strength at constant water content. Creep is basically caused by a continuing re-arrangement of the soil particles after the overburden pressure is fully supported by the soil skeleton, whereby the excess pore pressure has dissipated. Kuhn and Mitchell (1993) proposed that creep deformation is due to sliding between particles and that although the sliding is thought to occur at solid contacts, it is visco-frictional in nature and the sliding velocity at each contact depends on the ratio of tangential to normal components of contact force. Whether the creep strains in triaxial tests accelerate or not depends principally on the magnitude of the deviator stress compared to the strength or compressibility of the sample. Mitchell (1976) proposed the following general creep equation:

( )mrRa ttAe

dtd /αε

= (7.9)

where R=qt/qf delineates the deviator

stress level, tr is a reference time and A and α are solid constant parameters. When m=1 the strain rate continues to decrease with time, while the strain rate accelerates


towards failure when m<1. Considering m=1 and integrating Eq. 7.9 with εa = εa α at t = 1 then, ( ) tlAe n

Raa

ααεε =− (7.10)

is obtained. This is similar to the expression,

( )0/ ttlC na αε =∆ (7.11) representative of a one dimensional creep. Mukabi (1995), Mukabi and Tatsuoka, (1999a) reported results on the effects of aging in reconsolidation on deformation characteristics of various natural clays. The comprehensive research showed that time plays an important role in the stress-strain time history of clays. Based on comprehensive analysis of such results and considering that creep, which is predominantaly associated with secondary consolidation, contributes more significantly to the strength development of clay in comparison to thixrotropy and further assuming that ∆εa is purely a function of consolidation properties, then the following generalized relations were derived.

( )[ ]STCn

STC

STCSTCLTC

CSRAttKqKq

•−•

='/1 00

max0max φ

(7.12)

while,

( )[ ] STC

LTCSTC

nSTC

STCf

STCLTCf

qqCSRAttK

pKP

max

max00

0

'/1

''

••−

•=

φ (7.13)

and

( ) ( )

••∆∆−=

STCn

STCfca

STC

STCLTCf CSRAtttK

K'/1/

'00

0

φεφ (7.14)

where superscript LTC and STC

denote long term and short term consolidation respectively whereas t : LTC

time and to : STC time., for OC conditions (∆εa/∆t)fc

STC=1 7.3.3 Functions and parameters of reconstituted clays - The adverse effects of reconstitution of clays was briefly discussed in the preceeding section 7.3.2 of this paper.

From the analysis of various data based on the concepts of consolidation and shear stress ratio, the following correlations that can be useful in computing qmax, pf’ and φf’ from CUTC tests on reconstituted clays were derived.

( )( )( )RR

eRcf

Rfec

Re

I

e CSRAK

pq

pq

••−

••=

φµ

ηµ /

max

(7.15)

( )( )( )RR

eRcf

Rfec

Re

I

e CSRAK

pp

pq

••−

••=

φµ

ηµ / (7.16)

and

( ) [ ]( ) 1/

−•••−

••=

feRR

eRcf

Rc

ReI

pqCSRAK

ffφµ

φηµφ (7.17)

where superscripts I and R denote

“intact” and “reconstituted” respectively and µR

c=(q/p’)fR, ηc=(q/p’)c, and KR

cf = (σ’r/σ’a)R

ec.

7.4 Application in Determination of

Bearing Capacity Factors Bearing capacity factors, such as

Nγ,Nq and Nc in foundation design, are usually determined from the angle of shearing resistance. The shape and inclination factors are also known to vary with the angle of shearing resistance. Precise determination of this parameter and the cohesive intercept or apparent cohesion in case of clays or silts is therefore exceedingly important. In most cases and particularly in developing countries, these parameters are usually determined from drained or undrained isotropically consolidated triaxial tests due to the


complexity of simulating anisotropic conditions in laboratory testing. As demonstrated in the relations expressed in Eqs. 7.1 to 7.17, consolidation and shear stress ratio concepts are applied in circumventing this handicap. Due to consolidation and settlement, construction of foundations and structures on clays in terms of loading rate is an important aspect to consider. The same can be said about multi–stage construction of high embankments. In order to determine the appropriate rate of loading, Eqs. 7.3 to 7.5 derived from these concepts, can be effectively applied. Another problem that is usually experienced is the sampling of undisturbed samples. Retrieval of deep samples is not only expensive but is also associated with a certain amount of disturbance even in cases where the best samplers in the market are applied.

In order to erase the effects of disturbance, the SHANSEP method whereby reconstituted samples are adopted, was proposed. The functions proposed in Eqs. 7.15 to 7.17 are intended to greatly reduce the magnitude of this problem. These functions can also be particularly useful in determining strength and/or bearing capacity parameters for highway pavements, whereby the tests are usually conducted on reconstituted material. As reported in the preceding sections, aging in reconsolidation or long term consolidation is quite important in attempting to recover the stress–strain-time history of least disturbed clay samples. The relations proposed in Eqs. 7.12 to 7.14 can be applied in estimating the parameters related to this phenomenon since the actual performance of long term consolidation can prove to be time consuming and expensive. Long-term consolidation is also known to erase some of the effects of disturbance (Mukabi and Tatsuoka, 1999a), the magnitude of which depends on the length of consolidation time allocated. For foundations and structures constructed on clays, it is vital to consider the engineering aspects of over consolidation. The functions

proposed in Eqs. 7.6 to 7.8 can play an important role in solving this problem. 7.5 Application of Results in Modelling

and Prediction It is recognized that when using the

permissible stress method in design, a structure can fail due to excessive deformation caused by foundation settlement long before reaching a state of collapse due to failure in shear of the ground. In routine structures excessive total and differential settlement can often be avoided by choosing the safety factor at a sufficiently high value determined by experience, to avoid excessive settlement risks. However, in the case of heavy structures or those sensitive to differential settlement, a much more comprehensive and precise yet cost–effective method of calculation needs to be used in predicting the settlement. On the other hand, the limit state design as applied to foundations is that a structure and its supporting foundation should not fail to satisfy the design performance requirements because of various limit states. Design against the occurrence of the ultimate limit state is concerned with applying factors both to actions associated with loading and to the resistance of the ground associated with material factors so as to ensure that reaching the limit state is highly improbable. Serviceability limit states are concerned with ensuring that deformations or deflections do not damage the appearance or reduce the useful life of a structure or cause damage to finishes, non–structural elements, machinery or other installations in a structure. In whichever case, great emphasis is placed on limiting the occurrence of deformation. The ability to predict as precisely as possible, settlement due to loading is therefore exceedingly important.

Although sinking of foundations as a result of shear failure of the soil has been safe guard by the ultimate limit state computations or applying an arbitrary safety factor on the calculated ultimate bearing


capacity, it is still necessary to investigate the likelihood of settlements before the allowable pressures can be fixed. Employment of powerful analytical tools or methods to predict such settlement as precisely as possible cannot be overemphasized. The importance of so doing was demonstrated in section 2.2. In the following example, the effective application of the consolidation and shear stress functions compounded with the adoptation of properly determined elastic moduli in predicating reliable settlement values is demonstrated.

By applying the consolidation and shear stress ratio concepts to determine the stresses and linear elastic empirical relations to derive the elastic modulus, the resulting immediate settlement (quasi-elastic) under dynamic loading of the Trial Sections in stage 3 was computed from Eq. (7.18) and the results plotted in Fig. 7.4.

( )Psci xI

ExBP

21 µρ −∆= (7.18)

where, ρi= Initial quasi-elastic

settlement , ∆Psc=Net Surchange pressure, B=Width of foundation, E=Deformation Modulus, Ip= Influence factor (IP = 1.2) in this case, µ=Position Ratio. It can be noted from Fig. 7.4 that applying E50 as the deformation modulus in Eq. 7.18 grossly overestimates the settlement, while adopting the Emax yields a value that is in close agreement with the measured. In other words, prediction of settlement based on E50 can prove to be excessively costly in reference to the countermeasures that would have to be designed and applied.

8. SOME NEWLY DEVELOPED

QUANTITATIVE QUALITY CO-NTROL METHODS

8.1 Preamble

Due to the inherent nature of variability to every pavement design, material production and construction process, quality control during construction is of paramount importance. Good quality control however, requires vast experience and innovative thinking such that it is imperative for the engineer to have sound knowledge of the concepts, theories and principles that constitute the background of the ultimately specified quantities or parameters. Furthermore, the recognition of intrinsic variability in specification control is not a very simple task to accomplish despite the fact that the concepts may be statistically feasible. Developing statistically based quality assurance specifications requires innovative ways, enhanced research and/or a re-evaluation of historical construction records to obtain the relevant statistical parameters for typical construction that has performed satisfactorily and the one that may have proved problematic for the development of new solution systems.

Measured and field data collection would certainly serve no purpose if appreciable accuracy and confidence levels are not achieved. Accurate and precise definition of the boundary limits of specification control can prove to be costly if they are not properly considered or tailored for a specific project.

Fig. 7.4 Measured Vs. Predicted Settlement

0

1

2

3

4

5

6

7

0 1 2 3 4 5 6 7Measured Settlement (cm)

Pred

icte

d Se

ttlem

ent (

cm)

E50

1 : 1 Emax


The basic principles of some of the main quality control methods developed by

the Author previously on other project

modified to suit the design and construction specification requirements for the Addis Ababa~ Goha Tsion Project are briefly introduced in the subsequent sections. Numerous other interpretive methodologies, which are not introduced in this paper, have also been developed. 8.2 In-situ Moisture Content Vs.

Degree of Compaction Control 8.2.2 Formulae for correction of moisture content Vs. degree of compaction for low plasticity materials (crushed aggregates) - This method of correction takes into account the reciprocal relation between water content (wc), density (γ) and degree of compaction (Dc).

For low plasticity materials whereby PI < 6, the following generalized quasi-empirical equations may be applied.

( ) 100/

'

mc

sc

lc

optwwwflcu

cf DDwxxCxCnw

w−+

−=

γργ (8.1)

where, u

cfw = Moisture content correction

factor for DC>100, lcw = Moisture

content determined in the Laboratory, wfn = Constant derived from the relation between the natural and laboratory moisture contents,

γC = Density correction factor for laboratory and soil variability, Cw= Correction factor for moisture content, ρw= In-place wet density of soil, optγ = Maximum Dry Density

(MDD), scD =Specified Degree of

Compaction, mcD = Measured Degree of

Compaction. For cases where Dc < 100, the following equation may be applied:

( )( ) 100/

1'

mc

sc

lc

optwwwflcL

cf DDwxCxCnw

w−+

−−=

γργ (8.2)

Lcfw defines the moisture content correction

factor for Dc < 100. The corrected Degree of Compaction ( Cor

cD ) is then given by :

sc

wccfCor

c xDCxDw

Dγ

=. (8.3)

Where, .Cor

cD = Corrected degree of compaction, ul

cD = Standard upper limit degree of compaction, γC =Optimum density correction factor.

Considering some common and standard factors then , ,32.0=wfn ,93.0' =γC 89.0=wC

.977.0=γCand Based on the Specifications for this

Project for base course material, ulc

sc DandD %98= is determined as 102%.

Consequently, equations (8.1), (8.2) and (8.3) are simplified to the forms expressed in Eqs. (8.4), (8.5) and (8.6) respectively.

( ) 100/9826.0

mc

lc

optwlcu

cf Dww

w−+

−=

γρ (8.4)

While, ( )

( ) 100/98126.0

mc

lc

optwlcL

cf Dww

w−+

−−=

γρ (8.5)

and, 10002.1. xwD cf

Corc = (8.6)

Hence to correct for the

aforementioned variable parameters for base course material, Eqs. (8.4), (8.5) and (8.6) May be accordingly. 8.2.3 Formulae For Correction of Moisture Content Vs. Degree of Compaction for High Plasticity Materials (Subgrade, Embankment And Subbase) - For high plasticity materials whereby PI > 6, the following generalized quasi-empirical equations may be applied in all cases.


( ) 100/

'

mc

sc

lc

optwwwflc

cf DDwxxCxCnw

w−+

−=

γργ (8.7)

The corrected Degree of Compaction

( CorcD ) is then given by :

sc

wccfCor

c xDCxDw

Dγ

=. (8.8)

Considering some common and standard factors

,32.0=wfn 0.10.1,0.1' === γγ CandCC w Based on the Specifications for this

Project for subgrade material, ulc

sc DandD %95= is determined as 98%.

Consequently, equations (8.7) and (8.8) are simplified to the forms expressed in Eq. (8.9) and (8.10) respectively.

( ) 100/9532.0

mc

lc

optwlcu

cf Dww

w−+

−=

γρ (8.9)

and,

10098.0. xwD cfCorc = (8.10)

8.3 Mechanical Stability Analysis

In order to analyze the impact of mechanical stability on the bearing capacity Eq. (8.11) may be adopted.

( )opt

IIcr

Sopt

SRF BRBRxRff ±−= . (8.11)

where, S

RFf = Strength Ratio Parameter, S

optf . = Strength Ratio Parameter determined at the optimum Batching Ratio value, c

rR = Rate of Reduction of the post compaction strength ,

optIBR = Batching Ratio Index at optimum

value,

8.4 Quantitative Method of Evaluating Effect of Paving at Varying Grades of Slope

8.4.1 Preamble - In developing the method of evaluating effect of paving construction in negative upgrade slope, the factors in Table 8A were taken into consideration. TABLE 8A The four main influencing factors are stipulated in Table 8B. TABLE 8B 8.4.2 Rolling Resistance (Dynamic) Considering that,

LVGR Crr 254

2

=− (8.12)

then,

100254

2

−= ∑ LiVRG r

RCr (8.13)

1. Segregation of particles, flow characteristics, non-homogenity, contact pressure vibrational force, consistency, tractive force, sliding, Imperfect compaction, non-uniform thickness, impact on density, structural deficiency, differential deformation, localized flow and plastic failure.

2. Premature failure (craking or micro-cracking), non-

uniform inter-particle stress distribution, development and propagation of internal localized shear planes were also analyzed in relation to particle size, distribution, viscosity of bitumen, temperature, spreading rate, and state of inter-particle contact within a bituminous medium.

1) Grade effect on the strength and shearing resistance properties of the Asphalt Concrete

2) Damaging effect on the marshall

properties of the Asphalt concrete 3) Effect of rate of roadway superelevation 4) Effect of excitement frequency in relation

to micro-damage initiation due to construction equipment


where RCrG = Critical angle of slope in

relation to rolling resistance, Rr =Rolling resistance factor, V = Tractive velocity of construction equipment and L =Compaction distance 8.4.3 Damaging Effect (Static) -The damaging effect on the marshall properties of the asphalt concrete due to the critical angle of inclination is expressed as follows.

(8.14)

where eff

sv∆ =Damaging effect factor ,

limθ =Limiting grade of slope, θ i=Grade of slope. 8.4.4 Friction Factor (Dynamic) - The friction factor resulting from the dynamic component is computed as :

100127

2 eR

Vf F −= (8.15)

Where, fF=Friction factor, V=Velocity of construction equipment, f

CVG i =Critical grade of slope in relation to the friction factor, R=Radius of curvature

8.4.5 Effect of Excitement Frequency - Adopting the solution proposed by Housner (1963) for a half-sine wave acceleration pulse required for overturning a block and modifying it to that required to initiate slip of the surface mass; then the following equation is obtained for a value of ω that is small.

2lim 1)( ω

φθθ

gga p

cvs +−= (8.16)

where as=Acceleration to cause segregation, g = Force of gravity crθ =Critical grade of slope, limθ =Limiting grade of slope, φ p=Particle size (average), ω =Excitement

frequency propagated by the construction equipment

For a large value of ω , Eq. (8.16) can be represented by

( ) ( )limlim θµαθθθφω

υ −+=−≅= FKgpcrgpsa

(8.17) where, υ =Oscillatory velocity of construction equipment, θ=Angle between the hexagonal diagonal of an ideal particle with the normal line to the slip surface with an inclination of angle θlim., KF=Contribution of inter-particle friction factor, μ=Coefficient of friction between particle and slope. 9. ON-GOING AND PROPOSED RE-

SEARCH PROGRAMMES

Currently, various research programmes have been designed and are being undertaken by the same Research Group. The major on-gong or proposed research programmes are briefly introduced below. 9.1 Research Related to Black Cotton

or Expansive Soil Various engineering and scientific

concepts and theories have been adopted in designing the on-going testing regimes. Further research on the suction stress method and MC/Swell control interface layer technique is on-going. Embedment of various cheap and available material for drainage and reinforcement purposes are currently being considered under this topic. 9.2 Research Related to Intrusion of

Underlying Material Current research is designed to

determine not only the impact of varying infiltration material and geomaterials stabilized under various conditions, but is also concentrating on determining the relation between the history of dynamic loading effects, environmental factors and material intrusion or infiltration.

( )5.0

2

2limlim

tan1tantantantan

−

−=∆

θθθθθ iieff

sv


9.3 Research Related to Temperature and Seasonal Cycle Effects on the Bearing Capacity and Resilient Properties Enhanced testing regimes have been

designed to investigate various aspects related to this phenomenon. Some of the major objectives are to establish nomographs and correction factors that can be effectively applied under various environmental conditions. 9.4 Research Related to NDT/DT

Testing for the Evaluation of The Existing Condition of a Flexible Pavement Structure. It is the objective of this research that

an affordable yet versatile and effective method be establish through enhanced research on this subject and the consideration of various engineering and scientific aspects. 9.5 Proposed Research Related to

Consolidation and Shear Stress Functions for Foundation Design and Construction In order to develop more powerful

modelling and prediction techniques for foundation design and construction, the enhancement of this research is considered extremely important. This research is not on-going currently but proposals are being undertaken for the future. 9.6 Proposed Research to Relate Road

Surface Distress Condition and Deformation and Failure This research is to be proposed on

the basis of the existing preliminary knowledge on road surface distress conditions. The direct relationship of surface distress and shear failure in global terms is considered a rather complex subject. However, there could be a possibility of establishing this for isolated cases.

10 CONCLUSIONS This paper has dwelt predominantly on concepts of design and construction control

particularly tailored for developing countries that have been recently developed based on long-term research particular. The discussions emphasize that, in order to achieve an appropriate VE based design aimed realizing a cost-effective sound structure that would contribute to sustainable development, it is vital to undertake comprehensive research. The study has demonstrated that it is important to determine and evaluate as precisely as possible, design input parameters and construction control criteria based on research oriented concepts related to materials characterization, pavement and foundation structures as well as the employment of proper engineering judgment in most cases. 11 ACKNOWLEDGEMENTS The authors wish to express their sincere appreciation to the Japan International Cooperation Agency (JICA), Construction Project Consultants Inc., Kajima Corporation and Kajima Foundation for funding most of the study. The paper would certainly not have been completed without the input of Ms. Zekal Ketsella in typing, organizing and formating the manuscript. It is also important to mention the cooperation and assistance extended by the Ethiopian Roads Authority as well as the Ministry of Roads, Public Works and Housing, Kenya. The contributions of Mr. Sato of the University of Tokyo who designed most of the automated testing equipment, Eng. Kunioka, Mr. Horiuchi and Eng. Abebaw of Kajima Corporation Ethiopia, CPC Ethiopia Staff, Kajima Ethiopia staff including Ms. Meheret and Ms. Yodit as well as CPC Nairobi Staff, can not go without mention. The JICA Study Team and the former staff of Revtech Ltd. including Ms. Ruth Mukabi, Eng. Omollo, Mr. Sylvester Mukabi and Mr. Johnstone Mukabi, are indeed appreciated for their contribution during the research on the study on Rural Roads Improvement In Western Kenya.


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